Power control is essential in mobile wideband code division multiple access (WCDMA) communications systems to mitigate the near-far problem and to keep the rise over thermal (RoT) noise below an acceptable level. The near-far problem occurs when multiple transmitters transmit from different distances to a receiver, such that signals received from nearby transmitters cause greater interference and reduces the signal-to-noise ratio (SNR) of signals received from more distant transmitters. For example, this problem may arise when multiple wireless transmit/receive units (WTRUs) are communicating with a base station, or equivalently a Node B, in a wireless communications system.
Wireless communications systems based on code division multiple access (CDMA) technology, and in particular Third Generation Partnership Project (3GPP) WCDMA frequency division duplex (FDD) systems, therefore rely on a closed-loop power control mechanism to improve system performance. In typical WCDMA systems, uplink (UL) power is adjusted using regularly transmitted Transmission Power Control (TPC) commands from a Node B carried on the downlink dedicated physical channel (DPCH), or the fractional DPCH (F-DPCH), to a wireless transmit/receive unit (WTRU). The downlink (DL) power is adjusted using TPC commands from the WTRU carried on the uplink dedicated physical control channel (DPCCH) to the Node B. The uplink DPCCH also carries pilot bits in order to perform channel estimation at the receiver, such that the pilot bits enable accurate demodulation of the received signal. FIG. 1 shows a conventional usage of power-control loop channels in a WCDMA communications system. The WTRU and the Node B are each equipped with at least a processor, a transmitter and a receiver for use in transmitting and receiving communication signals over an established radio link including the F-DPCH or DPCH and the DPCCH. The processor operates according to a layered communications protocol, and generally includes a medium access control (MAC) layer component (layer 2), a physical (PHY) layer component (layer 1) and higher layer components (layer 3 and above) including, but not limited to, a radio resource control (RRC) layer component and a radio link control (RLC) layer component.
Transmission of the DPCCH on the uplink represents a significant power overhead, which not only reduces the battery power at the WTRU but also creates additional noise rise at the Node B. In addition, the transmission of the F-DPCH or DPCH on the downlink also contributes to power overhead and, even more importantly, consumes scarce CDMA code resource. In general, maintaining the power control loop is relatively costly and should be limited to when necessary, that is, when the WTRU transmits or receives data.
The 3GPP WCDMA FDD standards specify a number of modes and states of operations for a mobile WTRU to allow efficient use of power and radio resources. The amount of resource and power used by a WTRU depends on its current mode and state. In general, a WTRU in IDLE mode carries out cell search and uses very little power. Once in connected mode, a WTRU can be in one of four states: CELL_PCH state, URA_PCH state, CELL_FACH state and CELL_DCH state. In CELL_PCH state and URA_PCH state, the WTRU monitors the network for paging messages and communicates mobility messages to the network, and accordingly uses very small amounts of power and network resources. In CELL_FACH state, the WTRU continuously monitors the network for possible dedicated messages and therefore requires more power and network resources. A WTRU in CELL_FACH state can initiate data transmission on the random access channel (RACH), however, the RACH is only suitable for small amounts of data. In CELL_DCH state, all dedicated resources are allocated to the WTRU and the power control loop is maintained continuously. This is the most power-intensive state and it is designed for continuous transmission from and to the network and for carrying larger amounts of data. Details on the relationships between the different states are described in 3GPP Technical Standard (TS) 25.331 V7.5.0, which is incorporated herein.
In 3GPP high speed downlink packet access (HSDPA) Release 7, a number of features were introduced to reduce power control overhead associated with the transmission of voice over internet protocol (VoIP) and other sporadic traffic. In particular, the Discontinuous Transmission (DTX) and Discontinuous Reception (DRX) modes of operation were provided to allow the WTRU and Node B to reduce the frequency of power control and channel quality information (CQI) reporting, thereby increasing the number of users that can be supported in a cell. While these modes of operation are efficient for VoIP and similar types of traffic, DTX and DRX do not provide sufficient power-saving capabilities for traffic characterized by long periods of inactivity followed by short-length messages or bursty traffic. Examples of this type of traffic include virtual private network (VPN) keep-alive messages, uniform resource locator (URL) requests, internet browsing, file downloads and email. In these cases, during a long period of inactivity (also called a reading time), the power control loop, that is DPCCH and F-DPCH/DPCH, is still maintained even if no data is transmitted.
For these types of data traffic, it becomes inefficient to maintain the resource-consuming power control loop in CELL_DCH state. The power control overhead directly limits the number of users that can be serviced and translates into additional noise rise on the UL and additional interference levels on the DL. It also leads to inefficient use of the scarce battery resources of the WTRU. One option using the current technology of 3GPP HSDPA Release 7 is to move a WTRU from CELL_FACH state (or CELL_PCH state, Universal Terrestrial Radio Access Network Registration Area Paging Channel (URA_PCH)) to CELL_DCH state every time a new message needs to be transmitted, and the WTRU subsequently returns to CELL_FACH state (or CELL_PCH state, URA_PCH) state. However, this procedure would result in large signaling and resource overhead. In addition, maintaining the WTRU in CELL_FACH state would not be appropriate as the legacy RACH is not designed to transmit large amounts of data.
Additionally, an important resource in the HSDPA network is the Node B DL code space. The DPCH enhancements in 3GPP HSDPA Release 6 and Release 7 reduce the downlink power overhead associated with the Node B DL code space but fail to reduce the code overhead as the F-DPCH code resources are also assigned to DRX receiving WTRUs. As a result, the F-DPCH code resources cannot be used by other WTRUs.
Therefore, it is desirable to increase efficiency of the radio link by removing dependence on the DPCCH continuous transmission between WTRUs and Node Bs. Techniques for efficient use of scarce battery resources by reducing radio overhead in long periods of inactivity, reducing interference caused by control channels and increasing code availability on the DL, are also desirable.